Oxygen storage and oxygen concentration systems are used in the industrial gas, health care, and aerospace industries among others. In oxygen storage applications, oxygen capacity drives cost and utility. By designing systems that improve capacity without requiring changes in infrastructure, compression, or usage, an aspect of the current invention creates value. In oxygen concentration systems, power requirements and system form factor drive cost and utility. By designing systems that reduce the power required to separate the components of air while improving system design trade-offs such as form factor, an aspect of the current invention creates value.
It has been recognized that porous materials can be used to reduce the pressure of stored gases. This recognition is based on the concept of gas adsorption, wherein loading a vessel with solid material prior to charging with gas increases gas-solid contact area and distributes pressure more evenly throughout the system. The performance of these systems is tied to the internal surface area and pore uniformity of the solid material, properties that have been studied extensively in adsorbents such as zeolites and activated carbon.
Chang (U.S. Pat. No. 6,656,878) discloses an activated carbon adsorbent for the storage of oxygen gas at cryogenic temperature. The use of low temperature improves system performance, enabling this solution in specialized applications.
There is demand in the field for a system that could increase oxygen storage in high-pressure cylinders without requiring temperature control. Ideally, such a system would reversibly adsorb and desorb oxygen at room temperature, release the majority of its contents above 2 bar, and exhibit low thermal variance.
It has further been recognized that sieve beds selective for one air component over others can be utilized in pressure-swing adsorption (PSA) processes for air fractionation. The performance of these systems, as measured by product recovery given a purity target, is ultimately capped by the characteristics of the employed adsorbent.
Coe et al. (U.S. Pat. No. 5,417,957) teach using a lithium-exchanged zeolite, LiX, for the selection of nitrogen over other components in air. This material has been incorporated in PSA systems for the generation of 88-95% pure oxygen, or less commonly >80% pure nitrogen.
Nitrogen-adsorbing PSA processes are capped in oxygen purity due to the presence of non-adsorbing argon in air. If oxygen-adsorbing systems were used instead, the extract could be delivered as low-argon product. Oxygen is additionally a minor air component, requiring a smaller, lower-power system to process the same flow rate.
Due to material limitations, the design of oxygen-selective adsorption systems for bulk equilibrium separation has been infeasible. Equilibrium oxygen-selective materials bind too strongly, adsorb too little, diffuse too slowly, or otherwise exhibit properties that are not amenable to pressure-swing adsorption systems. The metallic structure of zeolites enables the exploitation of nitrogen's quadrupole moment, but disallows many oxygen-selective chemical mechanisms. Carbon molecular sieves are kinetically selective, but cannot be easily modified for equilibrium selectivity. Cross-linked polymers can embed many motifs, but cannot uptake enough gas for bulk separation applicability.
Munzner et al. (U.S. Pat. No. 3,979,330) teach using a carbon molecular sieve (CMS) to kinetically select oxygen over other components in air. Rather than thermodynamically preferring one gas to another, CMS PSA units exploit differences in diffusion rates between gases in air. These systems are used to produce nitrogen, stripping oxygen from the feed stream. However, the fact that CMS is not equilibrium selective increases the complexity of CMS-based PSA systems, deviating from the ideal Skarstrom-like cycle and increasing power requirements.
Mullhaupt et al. (U.S. Pat. No. 5,945,079) discuss the application of an oxygen binding mechanism known in the art to produce equilibrium oxygen-selective materials. This uses a reversible reaction mechanism embedded in a scaffold that has significantly lower capacity than zeolites, limiting its use in bulk air separation.
There is demand in the field for a high-capacity equilibrium-selective adsorption system for bulk air separation. Preferably, such a system would prefer oxygen to nitrogen, possess mechanical and chemical stability, reversibly adsorb and desorb at ambient conditions and be amenable to existing infrastructure.